A power generating assembly comprises for example a steam generator (nuclear boiler or reactor) associated with a turbine. An alternator which transforms the mechanical power of the turbine into electric power is generally coupled to this turbine.
A steam generator and turbine assembly is controlled by varying the rate at which heat is supplied to the generator or steam is supplied to the turbine, or by varying both of these factors simultaneously, to adjust the power supplied by the assembly to match the power required by the user.
This problem of the adjusting the power supplied by the assembly to the power requirement gives rise to difficulties.
Indeed, all transient operating states of the assembly such as a change in the rate of steam delivery or a change in the temperature of the steam give rise to mechanical and thermal stresses at various components of the turbine.
In particular, the disks of the first wheels of the turbine, or their extension in the central shaft of the rotor, are subjected to the greatest stresses; transient states which are too sudden can generate thermal stresses which are too high, causing the machine to break, particularly at the disks.
Until quite recently, operators of such assemblies only had empirical guide lines, based on experience, for operating such assemblies under operating conditions not envisaged in the operation manuals.
In practice, the said operators allowed for large safety margins; the result of this was that the power production assembly did not respond to the user's power requirements as quickly as would have been possible without exceeding the maximum permissible stresses for the equipment.
To facilitate the task of the operators of power production assemblies, means have been devised which make it possible to calculate at any instant the difference between the current stress at the most vulnerable point of the machine and the maximum permissible stress at the same point. But knowledge of this difference at a present instant is insufficient for operating an installation properly; indeed, the fact of having the value of such a difference available at a present instant does not make it possible to predict with certainty that the stress will remain acceptable without damage to the equipment in a near future, since the time constants of such assemblies are generally fairly large (about ten minutes).
The increased power demand of the modern world makes it essential that power production assemblies operate at the limits of their capabilities; in particular, it is necessary for an assembly to start up as quickly as the maximum permissible stresses allow. It is also desirable that each change in the power required should give rise to a transient state of the machine which lasts for as short a time as possible while guaranteeing that the stresses which the equipment undergoes remain within the permissible limits.